Of the reputed 619 UK-based fixed wing business (private) aircraft types, only 292, less than half, are on the British 'G' register. Of the rest of those parked up in the UK as their primary or nominal base, there are 132 on the Isle of Man register, 116 on the US 'N' register, six on the new Guernsey '2' register, one on the new 'ZJ' Jersey register, seven Bermudan, seven Cayman, one Maltese, two Luxembourg and the rest are general other European registers with the odd Nigerian or Saudi aircraft. Much of that is due to operational flexibility, but a lot is privacy and tax-related. Interesting though that the UK CAA only have oversight over the operation of less than half of the private aircraft actually based in the UK. Most of those non-G-registered aircraft of course are supposed to comply with the new Part-NCC regulations in August (safety compliance-related rules) so someone in the UK CAA needs the owner's phone and e-mail please - good luck!